use explicit returns to avoid checker confusion

The checker does not understand the idiom

if (except.reason < 0) {
   do_cleanups (whatever);
   GDB_PY_HANDLE_EXCEPTION (except);
}

because it doesn't realize that the nested 'if' actually has the same
condition.

This fixes instances of this to be more explicit.

	* python/py-breakpoint.c (bppy_get_commands): Use
	explicit, unconditional return.
	* python/py-frame.c (frapy_read_var): Likewise.
	* python/python.c (gdbpy_decode_line): Likewise.

gdb/python/python.c

@@ -653,7 +653,8 @@ gdbpy_decode_line (PyObject *self, PyObject *args)
     {
       do_cleanups (cleanups);
       /* We know this will always throw.  */
-      GDB_PY_HANDLE_EXCEPTION (except);
+      gdbpy_convert_exception (except);
+      return NULL;
     }
 
   if (sals.nelts)